1. Field of Invention
This invention is an economical method of recovering nitrogen as a product from biomass through the use of autotrophic organisms without the use of chemicals and with minimal energy inputs. The same process will convert low BTU biogas to high BTU biomethane gas.
2. Background Art
To control greenhouse gas induced global warming, methane gas emissions to the atmosphere from decomposition of waste residuals, such as food waste and animal manure, have been discouraged. However, anaerobic digestion (AD) is not a particularly economical process. The economics can be improved by: (1) exporting the energy as higher valued biomethane transportation fuel rather than electricity, and (2) co-digesting other waste residuals and thereby obtain a tipping fee—i.e., a fee charged to deliver municipal waste to a landfill, waste-to-energy facility or recycling facility. The first option is not particularly advantageous since the physical/chemical processes available to upgrade biogas to biomethane are expensive and have a cost approximately equal to the value of the natural gas or the biomethane produced. The second option, the co-digestion of additional substrates, increases protein conversion to ammonia and fugitive emissions of ammonia nitrogen to the atmosphere resulting in the creation of fine particulate matter (PM2.5), as well as biogenic NOx, N2O, and ozone. N2O is a powerful greenhouse gas at 310 times CO2 and the primary chemical responsible for stratospheric ozone depletion. Consequently, the discharge of ammonia nitrogen from anaerobic decomposition of organic and co-digested substrates results in significant adverse environmental and public health impacts.
Ammonia discharges from anaerobic digestion are strictly controlled in the EU and are expected to be controlled throughout the US. A number of technologies have been developed over the years to remove ammonia nitrogen from liquid waste streams through a variety of physical, chemical and biological methods. All of these processes are energy intensive, and expensive. Consequently, there is little incentive to curtail well-known and documented ammonia emissions from anaerobic digestion. It is expected that current and future regulation of ammonia nitrogen emissions will constitute a barrier to future implementation of renewable energy anaerobic digestion technologies.
All organic material contains 5% to 15% nitrogen (Redfield ratio, C:106, N:16, P:1). Digestion of high solid concentration, nitrogen rich, substrates such as food waste, algae, crop residues, whole ethanol stillage, poultry manure, etc. is hindered by ammonia toxicity created through the decomposition of protein. Ammonia toxicities to anaerobic microbes occur at concentrations exceeding 1,500 mg per liter, which will occur when the substrate solids concentrations exceed 6% solids, that contain 5% or more of nitrogen, and when the solids conversion to methane gas (bioconversion) exceeds 50%. Most high solids substrates such as poultry manure, food waste, and crop residues have solids concentrations exceeding 6% and nitrogen concentrations exceeding 5%. It is also desirable to convert more than 50% of the solids to gas and thereby maximize gas production. For poultry manure, dilution of the solids concentration from 40% to values as low as 6% solids requires substantial quantities of water that must be disposed after liquid solids separation. Such dilutions are typically not feasible. To overcome the dilution obstacle, a number of EU technologies such as the DRANCO, Valorga, and Kompogas have been developed to recycle the digestate liquid to the liquids/separation_step and thereby reduce the quantity of dilution water necessary to achieve lower solids concentrations. However, ammonia toxicity remains when the recycled digestate contains excessive ammonia, i.e. the ammonia is not removed.
In summary, the removal and reclamation of ammonia is important to improve the anaerobic digestion of organic residuals and minimize adverse environmental and public health impacts. The economical reclamation of ammonia as well as the production of high BTU gas will significantly improve the economics of AD.
3. Description of Related Art
Many strategies have been developed to remove and sequester ammonia nitrogen from the effluent of an anaerobic reactor. The basic strategy is to remove the ammonia from solution and form a second liquid or solid ammonium compound. Removing the ammonia from the digester effluent is normally preceded by decarbonization to remove CO2, followed by the addition of a chemical reagent, such as calcium, sodium or magnesium hydroxide to raise the solution pH and thereby shift the ionized ammonium to the unionized ammonia gas form (U.S. Pat. No. 4,104,131). Air containing a low concentration of ammonia is then used to strip the ammonia gas from solution. Steam has also been used to raise temperature, reduce the solubility
of carbon dioxide, increase the pH, and strip ammonia gas from solution by reducing the pressure and thereby decreasing the partial pressure of CO.sub.2 (U.S. Pat. No. 6,521,129).
High temperature (60-70.degree.C.) reduced pressure (0.25-0.75 bar) stripping has also been proposed (U.S. Pat. No. 6,368,849 B1). High temperature distillation or rectification of carbon dioxide and ammonia at an elevated temperature has been proposed (U.S. Pat. Nos. 4,710,300 and 6,368,849 B1). Membrane processes with decarbonization and pH adjustment have likewise been proposed. Pressurizing the digester contents and driving CO2 into solution has also been practiced. All these processes require a significant investment in energy for heat and pressure, and reagents for pH adjustment. Scale formation is a common problem if calcium or magnesium is used to adjust pH. Rectification or high temperature stripping requires the removal of most suspended solids prior to high temperature steam stripping or rectification.
Following ammonia stripping the ammonia can be sequestered through a variety of means. If high-temperature distillation is used to remove both carbon dioxide and ammonia, the uncontrolled formation of ammonium bicarbonate solids (scale) can be mechanically removed from the stripping unit (U.S. Pat. No. 4,710,300). If the ammonia is stripped with air or steam, anhydrous ammonia or aqueous ammonia can be formed at a reduced pH (U.S. Pat. No. 6,464,875, U.S. Pat. No. 5,702,572). If ammonia is stripped with air or steam ammonium salts can be formed through a reaction with a dilute acid (U.S. Pat. No. 6,521,129).
Biological processes have been used to remove ammonia nitrogen. They include aerobic nitrification and denitrification and the Anammox process whereby ammonia is anaerobically converted to nitrogen gas resulting in the loss of ammonia nitrogen's fertilizer value.
High temperature reduced pressure stripping, as well as distillation to remove both carbon dioxide and ammonia will improve the biogas quality since a portion of the carbon dioxide is removed from the gas stream under the high temperature conditions (U.S. Pat. No. 4,710,300). Improved gas quality has also been claimed when digesting a substrate having a high concentration of nitrogen through the formation of ammonium bicarbonate in solution (U.S. Pat. No. 7,160,456 B2). Also, biogas quality improvements have been claimed for processes that pass biogas through the digester liquid containing ammonia to form ammonium carbonate in the liquid slurry (U.S. Pat. No. 4,372,856, and U.S. Pat. No. 7,160,456).
A variety of processes are utilized to directly improve the BTU content of biogas. These processes involve the removal of carbon dioxide by high-pressure water scrubbing (U.S. Pat. No. 6,299,774), amine scrubbing, and membrane separation. Most of the systems involved high-pressure operation with significant capital and operation and maintenance costs. Biological processes have also been used such as acid phase anaerobic digestion (U.S. Pat. No. 5,529,692) where the CO2, formed in the acid phase, is separately removed from the predominately methane gas stream from the methane phase.
The economics of ammonia removal and sequestration, as well as the production of a high BTU biogas, can be improved significantly by operating a low pressure, low temperature, process that can remove substantially all of the ammonia while controlling the quality of the biogas produced. The process would be even more advantageous if it can be performed without the use of costly chemical reagents.